CORROSION BEHAVIOR OF RAPIDLY SOLIDIFIED MAGNESIUM-ALUMINUM-ZINC ALLOYS

Rapidly solidified magnesium alloys with 8 at%, 15 at%, and 20 at% Al and 1 at% and 3 at% Zn were fabricated by centrifugal atomization foll owed by hot extrusion. Microstructure of the alloys was composed of a fine-grain magnesium matrix (0.5 mu m) with beta-Mg17Al12 precipitates . Electrochemical and weight-loss tests were performed in berate and A STM D 1384 solution (chloride, carbonate, and sulfate). In both media, corrosion current of the alloys decreased with increases in aluminum or zinc content. In borate solution a passivating plateau was observed from the corrosion potential (E-corr) to E-corr + 1,200 mV. Current d ensity decreased with aluminum and zinc concentrations. Electrochemica l behavior of the synthesized matrix and precipitates was characterize d. Zinc increased E-corr of the two phases, with a corresponding decre ase of corrosion current. The same trend was noticed for aluminum but with a less dramatic effect. The corrosion mechanism was suggested res ult from galvanic coupling of the matrix and the second phase. The gal vanic corrosion, however, was reduced strongly by passivation of the m atrix as a result of the surrounding precipitates. The positive influe nce of rapid solidification (corrosion rate decreased 1 order of magni tude) was the creation of a fine, highly homogeneous microstructure th rough this fabrication process.